As the world is getting more digitalized, the development and usage of mobile applications have increased. Mobile digital services have become more efficient than the traditional non-digital, being a prominent part of today's society. Hence, studies on mobile applications' usability have become a focal focus for scholars, resulting in various usability attributes. However, due to pervasive digitalization, research scholars argue that there is a need for more research regarding the development and usability of mobile applications. Through a qualitative case study, the aim was to investigate how usability affects young adults' investment behavior using Avanza's mobile bank applications. Nielsen's theory of good usability consists of five attributes: learnability, effectiveness, memorability, error, and satisfaction. Based on eleven semi-structured interviews, our study reveals that. First, the usability attributes are strongly entangled and interdepended of each other. Second, good learnability creates good memorability, decreasing the number of errors and making the mobile application more effective. Third, two new attributes that complement Nielsens' theory of good usability have been identified, nudging and trust. Fourth and finally, in line with previous research scholars, our study reveals a need for further research on the role of usability in mobile applications in various contexts to enhance our understanding of how the design of a mobile application can affect the behavior of the users.